The present invention describes thick film photolithographic molds, methods of making thick film photolithographic molds, and methods of using thick film photolithographic molds to form spacers on a substrate. The thick film photolithographic molds preferably comprise an epoxy bisphenol A novolac resin. The present invention also describes sol gel spacers comprising sodium silicates and potassium silicates. The thick film photolithographic molds and sol gel spacers of the present invention can be used in flat panel displays, such as field emission displays and plasma displays.
FIG. 1 shows a cross sectional view of a portion of a prior art FED 100. FED 100 includes a cathode, or baseplate 102 and an anode, or faceplate 104. Baseplate 102 includes a substrate 106, a plurality of conical field emitters 108, an insulating layer 110, and a conductive grid layer 112. Insulating layer 110 is disposed over substrate 106, and grid layer 112 is disposed over insulating layer 110. Insulating layer 110 defines a plurality of void regions 114, and each emitter 108 is disposed over substrate 106 in one of the void regions 114. Grid layer 112 defines a plurality of apertures 116. Each aperture 116 corresponds to, and overlies, one of the void regions 114. The apertures 116 are positioned so that (1) the grid layer 112 does not obstruct a path 117 between the upper tips of the emitters 108 and the faceplate 104 and (2) a portion of the grid layer 112 is proximal to the tip of each emitter 108. Baseplate 102 also includes a plurality of conductive row and column lines 118 disposed between emitters 108 and substrate 106.
Faceplate 104 includes a glass plate 120, a transparent conductor 122, and a phosphor layer 124. Transparent conductor 122 is disposed on one major surface of glass plate 120, and phosphor layer 124 is disposed on transparent conductor 122. The faceplate 104 and baseplate 102 are spaced apart from one another and are disposed so that the phosphor layer 124 is proximal to the grid layer 112.
FED 100 also includes a plurality of spacers 130 disposed between the faceplate 104 and baseplate 102. The spacers 130 maintain the orientation between baseplate 102 and faceplate 104 so that the baseplate and faceplate are substantially parallel to one another. Outer walls (not shown) seal the outer periphery of FED 100 and the space between baseplate 102 and faceplate 104 is substantially evacuated. Since the space between faceplate 104 and baseplate 102 is substantially evacuated, atmospheric pressure tends to press baseplate 102 and faceplate 104 together. However, spacers 130 resist this pressure and maintain the spacing between baseplate 102 and faceplate 104.
FED 100 also includes a power supply 140 for (1) charging the transparent conductor 122 to a highly positive voltage; (2) charging the grid layer 112 to a positive voltage; and (3) selectively charging selective ones of the row and column lines 118 to a negative voltage.
In operation, voltages applied to the row and column lines 118, the grid layer 112, and the transparent conductor 122 cause emitters 108 to emit electrons 150 that travel along path 117 towards, and impact on, phosphor layer 124. Incident electrons 150 on phosphor layer 124 cause phosphor layer 124 to emit photons and thereby generate a visible display on faceplate 104.
The visible display of FED 100 is normally arranged as a matrix of pixels. Each pixel in the display is typically associated with a group of emitters 108, with all the emitters 108 in a group being dedicated to controlling the brightness of their associated pixel. For example, FIG. 1 shows a single pixel 160, with pixel 160 being associated with emitters 108a, 108b, 108c, and 108d. Pixel 160 could be a single pixel of a black and white display or a single red, green, or blue dot associated with a single pixel of a color display. Charging line 118a to a negative voltage simultaneously activates emitters 108a-d causing emitters 108a-d to emit electrons that travel towards and impact on phosphor layer 124 in the region of pixel 160. Normally, the row and column lines are arranged so that the emitters associated with one pixel can be controlled independently of all other emitters in the display and so that all emitters associated with a single pixel are controlled in unison.
The top of each spacer 130 contacts a portion of faceplate 104. Electrons emitted by emitters 108 can not impact phosphor layer 124 in the regions where the spacers 130 touch the phosphor layer 124. So, each spacer 130 creates a black, or dark, region of the display that can not be illuminated. The presence of dark regions created by spacers 130 does not significantly degrade the quality of the display of FED 100 as long as the area of the dark regions is small compared with the area of the illuminated pixels. Four or more spacers 130 are normally disposed around the periphery of each pixel. It is therefore important for the cross section of the spacers 130 to be relatively small compared with the area of each pixel. Ideally, the spacers 130 are characterized by a relatively high aspect ratio (i.e., the spacer""s height is larger than its width). Such a high aspect ratio (1) does not create dark regions large enough to degrade the display quality and (2) provides sufficient spacing between the baseplate 102 and faceplate 104 to permit electrons traveling from emitters 108 towards faceplate 102 to acquire sufficient energy to cause phosphor layer 124 to emit photons. The spacers 130 must also provide sufficient structural strength to resist the atmospheric pressure and thereby maintain the desired spacing between baseplate 102 and faceplate 104. It is also desirable for all spacers 130 to have exactly the same height so they can provide uniform spacing between the baseplate 102 and the faceplate 104. It is also important for the spacers to be properly aligned with respect to the array of pixels so the dark regions created where the spacers 130 contact the faceplate do not interfere with the display (e.g., it is desirable for the bottom of the spacers 130 to contact the grid layer 112 at selected locations that are between the apertures 116 and are equidistant from all the adjacent emitters). Since the spacers 130 are disposed within a vacuum, it is also important for the spacers to be formed from a vacuum compatible material (e.g., a material that does not outgas significantly).
Prior art spacers 130 are typically constructed using glass posts. After the posts are prepared they are bonded to the grid layer 112. Following this bonding, the faceplate 104 is fitted onto the posts. Functioning FEDs may be constructed using these techniques, however, these techniques have several disadvantages. For example, when the spacers are fabricated from glass posts, it is difficult to insure that every spacer has precisely the same height. Variation in spacer height degrades the parallel alignment between the faceplate and baseplate and thereby degrades the quality of the FED. Another problem with prior art spacer manufacturing techniques is that they do not permit precise alignment of the spacers with respect to the pixel array. As stated above, any deviation from the desired alignment can cause the dark regions created where the spacers contact the faceplate 104 to degrade the quality of the display. Ideally, the bottom of each spacer 130 contacts the grid layer 112 at a point that is equidistant from all the adjacent emitters, however, prior art spacer manufacturing techniques make it difficult to achieve this ideal. It would therefore be desirable to develop a new technique for fabricating spacers 130 for use in FEDs that provides improved control in manufacturability.
The use of porous, low-density xerogel materials for forming spacers in field emission flat panel displays and vacuum microelectronics is described in U.S. Pat. No. 5,658,832. In the methods described therein, a mold is placed on a substrate, such as the baseplate or faceplate in a field emission flat panel display. A mold release agent, such as glycerol, silicone or wax, is applied to the mold, and then a liquid xerogel precursor is placed in the mold. After the xerogel hardens, the mold is removed, such that a xerogel spacer is formed on the base plate or face plate. There are several problems associated with this process. First, the positive mold was created by using a saw to remove material from a four inch by four inch square substrate and thereby form an array of posts. Molds larger than four inches by four inches are generated by aligning an array of four inch by four inch tiles. These methods prevent the posts from being accurately positioned. Second, the posts formed by the techniques disclosed therein tend to be contaminated by air bubbles. That is, air bubbles tend to form near the base of the posts thereby weakening the posts as well as weakening the attachment between the posts and the substrate.
There is a need in the art for new and improved materials for spacers for flat panel displays, and for new and improved methods for forming the spacers on the face plates and base plates in flat panel displays. The present invention is directed to these, as well as other, important ends.
The present invention comprises methods of making and using molds for forming spacers for flat panel displays. Preferably, photolithography is used to make the molds. Molds are fabricated by depositing a thick film of photoresist, or photolithographic material, onto a support and by using photolithography to pattern the thick film photoresist into a shape for a desired mold. Positive molds define an array of posts, each of the posts corresponding to one spacer of the display that will ultimately be formed by the molding process. Negative molds define a plurality of apertures, each aperture corresponding to one spacer of the display that will ultimately be formed by the molding process. The photoresist can be patterned to form either positive molds (with arrays of posts) or negative molds (defining arrays of apertures).
One preferred thick film photoresist for use in fabricating the molds comprises an epoxy bisphenol A novolac resin, such as EPON(copyright) Resin SU-8. Methods are disclosed below for depositing a thick film of an epoxy bisphenol A novolac resin onto a support (e.g., a glass panel), for patterning the epoxy bisphenol A novolac resin layer into a desired shape for a mold, for improving the ability of the epoxy bisphenol A novolac resin to adhere to the support, for making the cured epoxy bisphenol A novolac resin less brittle or more resilient, and for planarizing a thick layer of uncured epoxy bisphenol A novolac resin.
The fabricated molds may be used to form spacers on a display. The mold is filled with a spacer material, brought into contact with a substrate (e.g., a faceplate of an FED), and the spacer material is allowed to cure into hardened spacers. After curing, the mold and substrate are separated leaving cured spacers attached to the substrate. Thereafter, the substrate and spacers may be dried and fired to further harden or cure the spacers.
Preferred materials for use as the spacer material are also disclosed below. Preferably, the spacer material is a liquid or gel that can be used to fill the apertures in a mold and that then cures into a hardened spacer. Preferred spacer materials comprise sodium silicate and potassium silicate. Other preferred materials comprise compositions comprising formamide, potassium silicate and sodium silicate.
Primary positive molds (fabricated using photolithography) may be used to form secondary negative molds. Such a secondary negative mold is preferably made by depositing a resilient material, such as latex, silicone, or plastic, over the primary positive mold so that the material conforms to the positive mold, allowing the material to cure, and then separating the cured resilient material from the positive mold. The resulting secondary negative mold can then be filled with a spacer material and placed in contact with a substrate so as to form spacers on the substrate.
Alignment of the mold and the substrate can be achieved by including an alignment template in the mold. One preferred method of achieving alignment is to provide some alignment marks near the border of a primary positive mold. A thin sheet of perforated metal (e.g., spring steel) that is flexible and resistant to permanent bending is then fabricated that replicates the alignment marks of the positive mold. The metal sheet is then positioned so that the alignment marks of the metal sheet are aligned with the alignment marks of the positive mold. Resilient material used for making the negative mold is then poured over the positive mold so that the resilient material encases the perforated portion of the metal sheet and so that the portion of the metal sheet including the alignment marks protrudes through the exterior of the resilient material. After curing, the resilient material is permanently bonded to the metal sheet. Thereafter, when the negative mold is used to apply spacers onto a substrate, the alignment marks of the metal sheet may be aligned with alignment marks on the substrate.
The molds and spacer materials constructed according to the invention are suitable for forming spacers on large flat panel displays. For example, spacers may be formed on displays with rectangular active areas measuring at least eight inches by ten inches.
Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description wherein several embodiments are shown and described, simply by way of illustration of the best mode of the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not in a restrictive or limiting sense, with the scope of the application being indicated in the claims.